Headlight low beams quit working

[Dave Wildrick]

During the recent “356 Ultimate Driving Tour” in Arkansas, the low beams on my headlights on the 65C coupe quit working, although the high beams did not.

I’m using Joe Leoni’s headlight relay kit and a standard Autopal H4 halogen headlight kit with separate bulbs and reflectors (bought from Jeffrey Fellman about 10 yrs ago)

When I returned to Houston, I pulled fuses 9,10,11, and 12. Jumping from the top of fuse 2 (always hot) to the bottoms of 11 and 12 caused the high beams to light, but I got no response jumping from the top of 2 to the bottom of either 9 or 10.

I replaced the fuses, removed the headlights and checked the sockets, and found that power was being delivered to both low beams with the headlight switch pulled all the way out and signal light switch in low beam position.

A check of the bulbs showed that the low beam filaments were blown in both bulbs, right and left; however, the high beam sections were still functional.

The sockets and wiring in the headlight buckets looked good and showed no evidence of broken insulation or other short circuit pathway.

I admit that these H4 bulbs are at least 5 years old, but how could both low beam bulb sections fail at the same time, without disturbing the high beams.

[David Jones]

It's called coincidence Dave. May be time to buy Jeffrey's new bulb kit and upgrade.

[Dave Wildrick]

It's called coincidence Dave. May be time to buy Jeffrey's new bulb kit and upgrade.

That's what I was thinking.

Meanwhile, I suppose I could just drive with the high beams on and p*ss people off.

[Jerry Henning]

Is your voltage regulator set a bit on the high side? This can shorten lamp life. Also, did you hit any major potholes while the lowbeams were on? A hard impact may have caused them to fail. Barring that, it was just their time, I would guess.

[Larry Brooks]

Is your voltage regulator set a bit on the high side? This can shorten lamp life. Also, did you hit any major potholes while the lowbeams were on? A hard impact may have caused them to fail. Barring that, it was just their time, I would guess.

I agree. Overdriving Halogen bulbs by 1.2 Volts(20%) will shorten their life by 60%. It also makes them very vulnerable to vibration and jolts. You probably lost one and the loss of that load would increase the effective voltage to the remaining bulb accelerating it's demise. The high beams are totally independent so barring a failure of the bulb envelope they should continue to burn until the excess voltage gets them too. Maybe even slightly longer due to the extra tungsten available to the Halogen Cycle from the low beams on the inside of the quartz envelope.

[Dave Wildrick]

Jerry and Larry,
Interesting points.

Here's the complete story.

A few years ago, I had Joe Leoni set my big black 6V regulator to a current limit of 23 amps.
Meanwhile, he recently completely rebuilt my old 6V generator with new armature, field coils, bearings, end bells--the works.
I road tested it for about 30 minutes, just driving on surface streets around Houston during the day. Also, I had not used the headlights in weeks, maybe months.
Unfortunately, the day I left Houston to begin Fred Uhlmann's Ultimate 356 Drive (May 8th), about 5 minutes down the freeway the red light came on. The fan belt was intact. So, oh crap what to do.
I drove on battery alone to Tyler, TX (about 250 miles N.) and at James Hughes' house, I was able to replace this generator with an unused rebuilt 12V generator (also rebuilt by Leoni a couple yrs ago), which Jim Learmonth was carrying with him as a backup.

I did not try the headlights during the entire 2000 mile-plus trip until the night before returning to Houston (May 13; Friday the 13th--hey, maybe that's it!), which was again in Tyler, TX. The low beam lights just did not come on.

So, I don't know for sure when they failed. I also don't know what running a 12V generator does to the current limit in the 6V regulator.
However, I do know that Joe Leoni told me he has run a 12V generator in his 6V car for over 6 years with no problems, because the regulator limits the voltage to 6V.
What does it all mean? I may never know.

[Larry Brooks]

Interesting. Have you tested your line voltage with the engine running at 2000rpm? An obvious place to check would be the headlight circuit.

[Jim Alton]

I can think of a couple things that might contribute to the low beams both going out while both high beams continued working:

1. Depending upon where you live you might use low beams a lot more than high beams. Here in Los Angeles it's pretty rare that you're on an open road and can use high beams.
2. Maybe the right and left low beams filaments didn't burn out at the same time and you only noticed when you had no low beams at all.
3. Maybe your car experienced some shock or vibration that caused the hot low beam filaments to fail while the cold high beam filaments weren't affected.

Of course I know no way to test any of these theories.

[Jon Schmid]

I have the opposite problem on one of my cars--the high beams won't work. I'm suspecting the dimmer switch is a good place to start to find out what's wrong.

[Dave Wildrick]

I have the opposite problem on one of my cars--the high beams won't work. I'm suspecting the dimmer switch is a good place to start to find out what's wrong.

Be sure to check the last 2 fuses, #11 & 12. Make sure the connections are tight in the fuse holders. On my other C coupe (1964), I had one fuse that kept getting loose, occasionally preventing one headlight high beam from coming on.

[Dave Wildrick]

Interesting. Have you tested your line voltage with the engine running at 2000rpm? An obvious place to check would be the headlight circuit.

I have not done this yet.
I'll check and let you know.

[Pascal Zundel]

Don't drive through Tyler, TX and you should be fine.

[Glen Hamner, Jr]

Got dielectric grease on all your connections?

[Jon Schmid]

Hi Dave,
Do you have the troubleshooting guide that the Registry gives (gave?) to every new member? Item 75, there are some good suggestions regarding headlight failures and remedies.

[Dave Wildrick]

Hi Dave,
Do you have the troubleshooting guide that the Registry gives (gave?) to every new member? Item 75, there are some good suggestions regarding headlight failures and remedies.

I'll bet I have it at home.
I'll have to take a look.
Thanks.